Ejection apparatus for printhead

ABSTRACT

An ejection apparatus for ejecting material from a liquid at a plurality of ejection locations ( 4 ) disposed in a row, has a plurality of channels ( 11 ) through each of which liquid flows in use to or from a respective ejection location at an open end of the channel. An ejection electrode ( 7 ) is disposed at each ejection location by means of which an electric field is created in use to cause the ejection of material from the liquid. An electrically conductive path ( 12 ) exists to each ejection electrode for supplying a voltage to the ejection electrode ( 7 ) in use. The channels ( 11 ) are isolated from one another and the electrically conductive paths ( 12 ) are separated from the channels over substantially the whole of their length.

The present invention relates to apparatus for ejecting material from aliquid and, more particularly, to an apparatus in which the methodemployed is generally of the type described in WO-A-93-11866, thedisclosure of which is incorporated herein by reference. In the abovepatent specification an agglomeration or concentration of particles isachieved in the printhead and, from the ejection location, theagglomeration of particles is then ejected onto a substrate, e.g. forprinting purposes.

In the case of an array printer, plural cells each containing anejection location, may be arranged in one or more rows. The presentinvention is directed towards novel constructions of such apparatus toimprove operation and enhance operability, and in this regard, referenceis made to our prior publications WO-A-97-27058, WO-A-97-27056,WO-A-97-27057 and WO-A-98-32609.

In those prior patent specifications there is disclosure of an arrayprinter in which a plurality of adjacent cells are formed between aseries of separating lands. Within each cell a further land carries anejection upstand which provides the ejection location for the material.As shown, for example, in FIG. 2 of WO-A-98-32609, side covers extendacross the tops of each of the cells to close the tops. Behind the sidecovers fluid which is fed to the printhead is held in a form ofreservoir or manifold and flows into the individual cells adjacent tothe ejection location, depending upon the volume of ejection from aparticular ejection location/cell.

The ejection mechanism of printers of this type, being at least partlyelectrostatic, and relying partly therefore on repulsion of theparticles from the ejection location, may result in a tendency forpigment particles (say in a pigmented ink) to flow away from a cell fromwhich ejection occurs to an adjacent cell from which ejection is notoccurring, by the mechanism of electrophoresis. A further difficultylies in the tendency of charged particles in the electric field toimpinge on surfaces of the electrodes, as a result of electrophoresis.Although the shear forces created by flow through such channels may actto dislodge particles from the surfaces, the shear force needs to begreater than the electrophoretic force and the charged particles need tobe moved away from the electric field before other particles impinge onthem. In order to improve print performances and long term printheadreliability, it would be desirable to reduce or eliminate the unwantedeffects of electrophoretic particle flow in the channels. In order tominimise the effects of the electrophoretic force one solution proposedin U.S. Pat. No. 5,754,199, is to drive the electrodes adjacent to theejecting electrode high first to force additional ink particles towardsthe ejecting electrode by means of electrophoresis. The object of thisis to pre-compensate for the fact that when a drive signal is applied tothe ejection electrode, ink particles will migrate away from the activeelectrode. This proposed approach will limit the printing speed.

According to the present invention there is provided an ejectionapparatus for ejecting material from a liquid at a plurality of ejectionlocations, the apparatus comprising a printhead having a plurality ofchannels through each of which liquid flows in use to or from arespective ejection location at an open end of the channel, each channelbeing one of two co-operating channels, one of which supplies liquid toa respective ejection location and the other of which removes depletedliquid from the same ejection location, the channels thus having acommon open end, an ejection electrode disposed at each ejectionlocation and by means of which an electric field is created in use tocause the ejection of material from the liquid, and an electricallyconductive path to each ejection electrode for supplying a voltage tothe ejection electrode in use, characterised in that the channels areisolated from one another and separated from the electrically conductivepaths over substantially the whole of the length of the electricallyconductive paths.

In the printhead design disclosed in WO 97/27058, the ink channels cancommunicate with each other over most of their length. Also according tothis design the electrodes communicate with the ink over the full inkpath in the channel. A consequence of this construction is that anelectrophoretic force acts on the ink particles that drives them towardsthe electrodes. If a sufficiently high fluid flow rate could be usedthen the shear force would be sufficiently high to prevent particlesbeginning to build up on the electrode structure. However, it would notbe practical to use such a high flow rate. The present inventionprovides a better solution in that by ensuring that the channels areisolated from one another; and the electrically conductive paths areseparated from the channels over substantially the whole of the lengthof the channels, the electrophoretic force is reduced and therefore alower shear force is required to prevent particles from building up onthe electrodes. Since a lower shear force is required the velocity ofthe flow can be reduced. The present invention also achieves the objectof U.S. Pat. No. 5,754,199 without requiring complex drive signals that,as stated earlier, will limit the print speed.

By this construction, the electrically conductive paths can be isolatedfrom the channels except in the immediate vicinity of the ejectionlocations, which isolation serves to reduce or prevent electrophoreticeffects causing a build-up of particles on the channel walls which wouldotherwise act to reduce the flow of material to the ejection locations.

Each channel is preferably one of a pair of co-operating channels, oneof which supplies liquid to a respective ejection location and the otherof which removes depleted liquid from the same ejection location, thechannels of each pair thus having a common open end. The channels ofeach pair may have longitudinal axes which are disposed at an angle toone another so that liquid is brought to the ejection location andremoved from it from the sides, the electrically conductive paths beingprovided substantially along a central axis bisecting the axes of thechannels. The channels may be formed along the edges of a pair ofprismatic bodies separate from a component which forms the ejectionlocations.

Preferably, the individual channels are separated from one another by aplurality of walls and isolation of the individual channels is achievedby closing the tops of the channels over a majority of their length,preferably by means of a pair of side covers, each of which is common toall of the channels on a respective side and which engages the walls.

At the open end of the channels, lands separate the ejection locationsfrom one another. Protrusions preferably form the ejection locations andare also defined by lands between the lands which separate individualejection locations from one another, the protrusions of the ejectionlocation-defining lands being smaller in width and defining, on eachside thereof, passages for liquid flow between the ejectionlocation-separating lands. In order to separate the ejectionlocation-defining lands from the ejection location-separating lands,spacers may be provided on the flanks of the ejection location-defininglands at least over most of their length, the separators being formedintegrally with the protrusion-defining lands, the channel-separatinglands or both. The lands and separators are preferably formed by acomponent located between the prismatic bodies in which the channels areformed.

By this construction, each channel may be of increased length incomparison with the prior designs mentioned above and by providing sidecovers which close the individual channels, fluid separation betweenthem is provided.

Preferably, the separators are metallised to provide the electricallyconducting paths to the ejection electrodes.

Preferably, each electrically conductive path is disposed so as to beout of contact with the liquid, in use, over substantially the whole ofits length. Each electrically conductive path may be disposed so as tobe in contact with the liquid, in use, solely at the region of thecorresponding ejection location.

One example of an apparatus according to the present invention will nowbe described with reference to the accompanying drawings in which:

FIG. 1 is a perspective view, from the front and one side, of part of anink jet printhead;

FIG. 2 is a perspective view, again from the front, in close-up;

FIG. 3 is a longitudinal section through the printhead;

FIG. 4 is a perspective view of the printhead showing supporting bodiesattached to each side and,

FIG. 5 is a further perspective view showing side covers attached toeach of the supporting bodies.

FIGS. 1 and 2 illustrate the construction of the individual cells 2 of aprinthead 1, individual cells 2 being separated by separating lands 3and each cell including an ejection location defined by a protrusion 4formed at the end of a corresponding land 5 disposed substantiallycentrally between the cell-defining or ejection location/cell-separatinglands 3 on each side.

Ejection electrodes 7 are formed by selective metallisation of surfacesof the cell-separating and protrusion-forming lands 3,5. Each of theprotrusion-supporting lands 5 is separated from the adjacentcell-separating lands by a spacer 6 (only one of which can be seen ineach cell 2 in FIGS. 1 and 2, because of the perspective), the surfaceof each spacer likewise being metallised to provide an electricallyconductive path or track 12 to the respective ejection electrodes 7 fromthe rear of the printhead (best seen in FIG. 3).

As is clearly seen from FIGS. 1 and 2, at the front of each cell thelands 5 are tapered at an acute angle and support the ejection locationprotrusion 4 at their front edge, the cell-separating lands 3 beingtapered at a larger angle as shown.

As is best seen in FIG. 4, liquid supply and removal channels 11 whichrespectively supply and remove liquid from the cells 2 and which in partdefine the cells 2, are provided between walls 8, integrally formed withone another on a supporting body 9 generally triangular in shape whenviewed from the end of the printhead. The walls 8 are disposed inregistration with the cell-separating lands 5 in order to define thechannels 11 over their whole length. FIG. 5 illustrates side covers 10which close the channels 11 formed between the walls 8.

The precise shape of the protrusions 4, defining the ejection locations,will depend upon the application, the intended liquid with which theprinthead is to be used, its operating conditions etc. In the exampleshown, the protrusions 4 are simple triangular forms or upstands formedon the front edge of the lands 5, but of lesser thickness. In theexample shown, the lands 3, 5, upstands 4, spacers 6, wings 8 and sidecovers 10 are formed from a ceramic material.

It will be appreciated, that, by providing the electrically conductingtracks or paths 12 along the centre of the printhead (see FIG. 3), wellseparated from the flow channels 11, the channels are separated from theelectrically conducting paths 12 over substantially their whole length,the electrically conducting paths and the channels only being in contactwith one another at the ejection locations, ie where the electricallyconducting paths 12 provide the electrodes 7.

For a printhead with channels that are 100 μm wide and with a length of10 mm each side of the ejection electrode region, the ink flow rate istypically 0.01 ms⁻¹ in the centre of the channels. With ejection voltagepulses being of the order of 1000V, if one channel is, on average,driven far more often than its neighbour, a typical electrophoreticelectric field of 5×10⁴ Vm⁻¹ will be generated in the ink between theejection electrodes. A typical value of the electrophoretic force on theparticles will be 10⁻¹³ N and the resulting electrophoretic velocitieswill be around 10⁻⁵ ms⁻¹. The flow of ink through the channels willgenerate a shear force on the particles of 5×10⁻¹³ N. As this isconsiderably greater than the electrophoretic force, any particle pushedonto the walls of a channel will be swept along the channel by the flow.This results in particles closest to the channel wall passing the first20 μm of each electrode in about 0.1s. The significance of this is that,due to electrostatic screening, it is within the first 20 μm thatparticle deposition on the electrode tends otherwise to occur as aresult of particles being pushed onto the electrode in that region at arate of about one layer every 2s. The fact that the particles closest tothe channel wall are swept past the first 20 μm of electrode beforeanother layer of particles is laid down eliminates such particledeposition.

1. An ejection apparatus for ejecting material from a liquid at a plurality of ejection locations, the apparatus comprising a printhead having a plurality of channels through each of which liquid flows in use to supply liquid to a respective ejection location at an open end of the channel, and a plurality of channels through each of which liquid flows in use to remove depleted liquid from the respective ejection location at an open end of the channel, wherein each channel is one of a pair of co-operating channels, the two cooperating channels thus having a common end, an ejection electrode disposed at each ejection location and by means of which an electric field is created in use to cause the ejection of material from the liquid, and an electrically conductive path to each ejection electrode for supplying a voltage to the ejection electrode in use, wherein the channels are isolated from one another and separated from the electrically conductive paths over substantially the whole of the length of the electrically conductive paths.
 2. Apparatus according to claim 1, wherein the channels have longitudinal axes which are disposed at an angle to one another so that liquid is brought to the ejection location and removed from it from the sides, the electrically conductive paths being provided substantially along a central axis bisecting the axes of the channels.
 3. Apparatus according to claim 2, wherein the channels are formed along the edges of a pair of prismatic bodies separate from a component which forms the ejection locations.
 4. Apparatus according to claim 1, wherein the individual channels are separated from one another by a plurality of walls and isolation of the individual channels is achieved by closing the tops of the channels over a majority of their length.
 5. Apparatus according to claim 4, wherein the individual channels are closed, over a majority of their length, by means of a pair of side covers, each of which is common to all of the channels on a respective side and which engages the walls.
 6. Apparatus according to claim 4, wherein, at the open end of the channels, lands separate the ejection locations from one another.
 7. Apparatus according to claim 6, wherein protrusions are also defined by lands between the lands which separate individual ejection locations from one another, the protrusions of the ejection location-defining lands being smaller in width and defining, on each side thereof, passages for liquid flow between the ejection location-separating lands.
 8. Apparatus according to claim 7, wherein, in order to separate the ejection location-defining lands from the ejection location-separating lands, spacers are provided on the flanks of the ejection location-defining lands at least over most of their length, the separators being formed integrally with the protrusion-defining lands, the channel-separating lands or both.
 9. Apparatus according to claim 7, wherein the lands and separators are formed by a component located between prismatic bodies in which the channels are formed.
 10. Apparatus according to claim 9, wherein the separators are metallised to provide the electrically conducting paths to the ejection electrodes.
 11. An ejection apparatus according to claim 1, wherein each electrically conductive path is disposed so as to be out of contact with the liquid, in use, over substantially the whole of its length.
 12. An ejection apparatus according to claim 1, wherein each electrically conductive path is disposed so as to be in contact with the liquid, in use, solely at the region of the corresponding ejection location.
 13. An ejection apparatus for ejecting material from a liquid at a plurality of ejection locations, the apparatus comprising a printhead having a plurality of channels through each of which liquid flows in use to supply liquid to a respective ejection location at an open end of the channel, and a plurality of channels through each of which liquid flows in use to remove depleted liquid from the respective ejection location at an open end of the channel, wherein each channel is one of a pair of co-operating channels, the two cooperating channels thus having a common end, an ejection electrode disposed at each ejection location and by means of which an electric field is created in use to cause the ejection of material from the liquid, and an electrically conductive path to each ejection electrode disposed so as to be out of contact with the liquid, in use, over substantially the whole of its length, for supplying a voltage to the ejection electrode in use, wherein the channels are isolated from one another and separated from the electrically conductive paths over substantially the whole of the length of the electrically conductive paths.
 14. An ejection apparatus for ejecting material from a liquid at a plurality of ejection locations, the apparatus comprising a printhead having a plurality of channels through each of which liquid flows in use to supply liquid to a respective ejection location at an open end of the channel, and a plurality of channels through each of which liquid flows in use to remove depleted liquid from the respective ejection location at an open end of the channel, wherein each channel is one of a pair of co-operating channels, the two cooperating channels thus having a common end, an ejection electrode disposed at each ejection location and by means of which an electric field is created in use to cause the ejection of material from the liquid, and an electrically conductive path to each ejection electrode disposed as to be in contact with the liquid, in use, solely at the region of the corresponding ejection location for supplying a voltage to the ejection electrode in use, wherein the channels are isolated from one another and separated from the electrically conductive paths over substantially the whole of the length of the electrically conductive paths.
 15. An ejection apparatus for ejecting material from a liquid at a plurality of ejection locations, the apparatus comprising a printhead having a plurality of channels through each of which liquid flows in use to or from a respective ejection location at an open end of the channel, each channel being one of two co-operating channels, one of which supplies liquid to a respective ejection location and the other of which removes depleted liquid from the same ejection location, the two cooperating channels thus having a common open end, an ejection electrode disposed at each ejection location and by means of which an electric field is created in use to cause the ejection of material from the liquid, and an electrically conductive path to each ejection electrode for supplying a voltage to the ejection electrode in use; wherein the channels are isolated from one another and separated from the electrically conductive paths over substantially the whole of the length of the electrically conductive paths; and wherein the channels have longitudinal axes which are disposed at an angle to one another so that liquid is brought to the ejection location and removed from it from the sides, the electrically conductive paths being provided substantially along a central axis bisecting the axes of the channels.
 16. Apparatus according to claim 15, wherein the channels are formed along the edges of a pair of prismatic bodies separate from a component which forms the ejection locations.
 17. Apparatus according to claim 15, wherein the individual channels are separated from one another by a plurality of walls and isolation of the individual channels is achieved by closing the tops of the channels over a majority of their length.
 18. Apparatus according to claim 17, wherein the individual channels are closed, over a majority of their length, by means of a pair of side covers, each of which is common to all of the channels on a respective side and which engages the walls.
 19. Apparatus according to claim 17, wherein, at the open end of the channels, lands separate the ejection locations from one another.
 20. Apparatus according to claim 19, wherein protrusions are also defined by lands between the lands which separate individual ejection locations from one another, the protrusions of the ejection location-defining lands being smaller in width and defining, on each side thereof, passages for liquid flow between the ejection location-separating lands.
 21. Apparatus according to claim 20, wherein, in order to separate the ejection location-defining lands from the ejection location-separating lands, spacers are provided on the flanks of the ejection location-defining lands at least over most of their length, the separators being formed integrally with the protrusion-defining lands, the channel-separating lands or both.
 22. Apparatus according to claim 20, wherein the lands and separators are formed by a component located between prismatic bodies in which the channels are formed.
 23. Apparatus according to claim 22, wherein the separators are metallised to provide the electrically conducting paths to the ejection electrodes. 